This non-provisional application claims priority under 35 U.S.C. xc2xa7119(a) on Patent Application No. 2001-90808 filed in Japan on Mar. 27, 2001, which is herein incorporated by reference.
1. Field of the Invention
This invention relates to an exhaust emission control system of an internal combustion engine having a function of regenerating an exhaust emission control device capable of trapping harmful matters included in exhaust.
2. Description of Related Art
Examples of prior art relating to such an exhaust emission control device are disclosed in Japanese Laid-Open Patent Publications Nos. 4-47119, 61-79814 and the like.
The exhaust emission control system disclosed in the Japanese Laid Open Patent Publication No. 4-47119 supplies fuel into an exhaust passage at a position upstream of a filter, and regenerates the filter by oxidizing particulate matters deposited on the filter by means of combustion heat of the fuel. In particular, this exhaust treatment system provides control such that the quantity of fuel to be supplied is increased with an increase in the oxygen flow in the exhaust and decreased with a rise in the exhaust temperature. Thus, this exhaust treatment device enables the combination temperature of the supplied fuel to reach a predetermined temperature during regeneration of the filter, so that the particulate matters can be oxidized without overheating the filter.
On the other hand, the exhaust treatment system disclosed in Japanese Laid-Open Patent Publication No. 61-79814 heats a filter by means of an external heat source such as an electric heater, and oxidizes particulate matters by resulting heat. In particular, if an engine starts idling during regeneration of the filter, this exhaust treatment system provides control such that the oxygen concentration is controlled to be equal to or lower than a predetermined upper limit in the case where the temperature of the filter exceeds a predetermined temperature and the oxygen concentration of exhaust exceeds the predetermined upper limit. Thus, this exhaust treatment system is able to prevent the filter from overheating due to the rapid oxidization of the filter even if the filter is sufficiently heated with a small amount of particulate filters being deposited thereon.
Although the above-mentioned exhaust treatment system disclosed in the Japanese Laid-Open Patent Publication No. 4-47119 decreases the quantity of fuel to be supplied with a rise in the exhaust temperature during the regeneration of the filter, the system merely sets the combustion temperature of the fuel to a predetermined value without taking into consideration the overheat of the filter due to the combustion of the particulate matters. Specifically, even if the combustion temperature of the fuel is maintained at a predetermined value during the regeneration of the filter, the filter may be damaged by melting due to the combustion heat of the fuel in the case where the particulate matters are rapidly combusted.
On the other hand, the exhaust treatment system disclosed in the Japanese Patent Laid-Open Publication No. 61-79814 controls the oxygen concentration to be equal to or lower than the predetermined upper limit (e.g. 4%) to thus prevent the rapid combustion of the fuel. However, the combustion proceeds slowly while the oxygen concentration is low, and therefore, it takes a long period of time to regenerate the filter.
It is therefore an object of the present invention to provide an exhaust emission control system of an internal combustion engine, which is capable of preventing the exhaust emission control device from being damaged by melting due to overheating during regeneration, and reducing the time required for regenerating the exhaust emission control device.
To accomplish the above object, an exhaust emission control device of an internal combustion engine includes an exhaust emission control device disposed in an exhaust passage of the internal combustion engine and is capable of trapping harmful matters in exhaust, an exhaust flow rate detecting device that detects a flow rate of exhaust flowing through the exhaust passage, and a concentration control device that exhaust emission control device according to the flow rate of the exhaust when the exhaust emission control device is regenerated.
Usually, as the combustion of harmful matters such as particulate matters proceeds during regeneration of an exhaust emission control device such as a filter, the exhaust emission control device temperature is raised by heat resulting from the combustion. If the combustion further proceeds, the exhaust emission control device may be damaged by melting due to an excessive rise in the temperature although the time required for regeneration is reduced. However, if the combustion proceeds slowly, it takes a long time to regenerate the filter.
Thus, there is a trade-off relationship between prevention of the exhaust emission control device from melting and reduction of the time required for regeneration. The exhaust emission control system effective for attaining these objects has been developed with removal of heat by an exhaust flow in mind. More specifically, even in the case where the particulate matters are actively combusted during the regeneration of the exhaust emission control device, if the flow rate of exhaust passing through the exhaust emission control device is high, the exhaust flow accordingly removes a large amount of heat to prevent the temperature of the filter from rising excessively. On the other hand, if the flow rate of exhaust is low, the exhaust flow removes only a small amount of heat, and in is therefore necessary to control the combustion of the particulate matters in order to prevent the exhaust emission control device from overheating. Therefore, with the relevancy of the exhaust flow rate to the amount of removed heat in mind, the exhaust emission control device according to the present invention actively controls the oxygen concentration according to the detected exhaust flow rate to thereby control the degree to which the combustion of the particulate matters proceeds with in such a range as not to overheat the exhaust emission control device.
Since the regeneration of the exhaust emission control device is completed within a short period of time while preventing the filter from being damaged by melting, the exhaust emission control performance and reliability of the exhaust emission control device are considerably improved.